The present invention generally relates to hearing aids and more particularly relates to open-ear type devices that allow incident sound to reach the eardrum directly.
Hearing aids typically consist of a microphone, a signal processor, and an output transducer (sometimes called a “receiver”). The output transducer is placed in the ear canal and can be part of a housing that either leaves the ear canal partially open (i.e., acoustically transparent) or seals the canal completely. Open-ear devices are generally preferred over closed-ear devices by users and are recommended whenever possible for persons with mild or moderate hearing loss. (Open hearing aids have inherent limitations in the amount of gain they can provide, and thus are not well suited for persons whose hearing loss is severe.)
One advantage of open-ear devices is comfort: the soft tip of open-ear designs is less irritating and easier to adapt to than hard-shell closed-ear inserts. There is also less risk of infection or impaction by cerumen (ear wax). No custom ear-mold is required, which substantially decreases the fitting time and allows such hearing aids to be used off the shelf with only minor modifications. Also avoided is the occlusion effect, where the closed ear canal forms a resonant chamber that boosts low frequency sounds generated by the user (such as speech or chewing), causing the user's voice to sound unnatural and boomy. The occlusion effect is one of the primary reasons cited when users reject closed-style hearing aids.
Open-ear designs also allow better processing in complex acoustic environments, because they allow the incident sound to be heard at frequencies where the hearing aid provides no amplification. For example, a hearing aid fit to a high frequency hearing loss (above 1 kHz) doesn't need to amplify low frequencies. The incident sound is worth preserving whenever possible because it carries perceptual cues required for localizing sound sources and rejecting background noise. Such perceptual cues include interaural timing differences, interaural loudness differences, and phase effects.
Despite their advantages, open ear hearing aids have significant drawbacks. One drawback comes from artifacts and distortion that can be produced at the eardrum by the combination of incident and amplified sound at frequencies amplified by the hearing aid. These artifacts and distortion are often noticed by users and result in dissatisfaction that leads many to stop using their hearing aids after a short period of time.
One artifact results from the latency of the hearing aid, that is, the time delay between when a sound is sensed at the microphone and when it is converted to an acoustical sound wave at the hearing aid's output transducer. For modern digital hearing aids, the latency is 3-7 milliseconds; older analog hearing aids have a latency around 1-2 milliseconds. When both the incident and amplified sounds are similar in level, non-zero latency causes comb filtering, a form of spectral distortion. Comb filtering is characterized by a series of regularly spaced spectral peaks and dips in the sound pressure at the eardrum. For longer latencies, the first dip is at a lower frequency and hence a larger portion of the frequency spectrum is affected. Shorter latencies produce less extensive comb filtering. The human ear is very sensitive to this kind of artifact; latencies shorter than 8 milliseconds are perceived as tone coloration, while longer latencies can be perceived as echos, beating, or tone coloration depending on the relative loudness of the delayed sound.
Another recombination artifact arises from phase distortion in the amplified sound. This also produces a structure of spectral dips and peaks; wherever frequencies are 180 degrees out of phase, they recombine destructively and create a dip, while those in phase add constructively, creating a peak. Since phase distortions are often spread non-uniformly over the frequency spectrum, this kind of artifact is potentially much less regular than latency artifacts. The source of phase distortion can be any component in the signal path: the microphone, signal processing components, or the output transducer (loudspeaker).
The above-mentioned artifacts result in spectral distortions to the perceived sound readily apparent to even untrained listeners. In addition to these spectral distortions, hearing aids also distort the phase information when the amplified signal is much louder than the incident signal. It is believed that such phase distortions are themselves noticeable. Recent evidence suggests that phase is used for many tasks, including source localization, speech encoding, and detection of phase modulation.
The present invention addresses the drawbacks associated with conventional open ear hearing aids. It substantially mitigates the artifacts and distortion problems that exist in open ear hearing aids, and substantially eliminates the source of user dissatisfaction with this type of hearing aid design. The invention allows the user to enjoy the well-known benefits of open-ear designs without suffering the perceptible distractions commonly associated with such designs.